EP0266808B1

EP0266808B1 - Process for delaying and controlling the formation of gels or precipitates derived from aluminum and corresponding compositions together with its applications particularly those concerning the operation of oil wells

Info

Publication number: EP0266808B1
Application number: EP19870201737
Authority: EP
Inventors: Alan Parker
Original assignee: PUMPTECH NV
Current assignee: PUMPTECH NV
Priority date: 1986-10-08
Filing date: 1987-09-11
Publication date: 1992-03-18
Anticipated expiration: 2007-09-11
Also published as: EP0266808A1; FR2604989B1; DE3777534D1; FR2604989A1

Description

This invention covers compositions which make it possible to retard and control the formation of gels or precipitates derived from aluminum, and has applications for the operation of oil wells.
During the production of hydrocarbons from underground formations, it is often desirable to reduce the permeability of certain zones in the formation. This is the case, for example, when a formation neighboring on the production well is likely todirect water into the well. Another example is where zones of high permeability are neighboring on the injection wells.
One method of reducing the permeability of certain undergound zones consists in precipitating a product in-situ. This technique is well understood and is usually employed by the placing of two incompatible fluids inside the reservoir. This leads to the formation of a precipitate, giving reduced permeability wherever the two fluids come into contact and mix inside the reservoir. The specialist is referred to US Patents, Nos. 3,837,400 -4,031,958 and 4,304,301.
A second possibility lies in the introduction of a fluid designed to react with a particular component of the reservoir, such as inherent water in the latter, to form an insoluble precipitate. The specialist is referred to US Patent No. 3,837,400.
Another technique consists yet again in introducing a fluid with the intrinsic potential of creating a precipitate or gel, such formation however requiring retardment by some means. The specialist is referred to US Patents, Nos 4,413,680 and 3,614,985.
In order to master the operation, it is essential that two parameters are controlled: the time required for the formation of the gel or precipitate and the morphology of the solid phase subsequently produced. This is because the time taken for the reaction determines the possible degree of penetration into the formation, while the morphology of the solid phase defines the degree by which the permeability of the formation is reduced.
US Patent No. 3,614,985 describes the precipitation of metal hydroxides such as chromium hydroxide or aluminum hydroxide in a homogeneous solution.
Numerous simple salts of aluminum III (such as aluminum chloride or aluminum nitrate) are soluble in slightly acid mediums, but form insoluble aluminum hydroxides when the pH level goes beyond about 5.
The previously cited patent describes the delayed formation of aluminum hydroxide-type compounds by employing a solution consisting of a suitable aluminum salt, and an activator. The activator is used to slowly increase the pH level within the temperature ranges normally found at the well bottom. This in turn leads to the slow formation of an insoluble hydroxide phase. As stated in this document, a suitable activator is urea, which undergoes hydrolosis at high temperatures to form ammonia which consequently raises the pH of the solution. The solid aluminum hydroxide precipitate phase has a tendency to remain amorphous, with the consistency of a gel.
The only example of a practical system described in the above mentioned patent concerns the precipitation of aluminum hydroxide from aluminum chloride. However, the way it is described, this solution does not have a practical application in oilfield operations because of both safety and technical constraints. In fact, aluminum chloride in solid form reacts violently with water, giving off heat and hydrochloric acid vapors.
Such a system would therefore be extremely dangerous to handle in the field. In addition, the use of concentrated solutions of aluminum chloride is not desirable on account of the volume of fluid required and the need to employ a corrosion inhibitor to protect the fluid-metering pumps. It is an established fact that corrosion inhibitors often affect well treatment.
Typical treatment solutions (approximately 0.05 to 0.15 M A1 3+) have a pH of about 3 and, as recommended in the above patent, it is preferable to adjust the pH to about 4. In fact, with a pH of 3, fluid interaction with the carbonate present in the porous medium is very rapid and limits peneration of the fluid into the matrix. Even in the case of a very pure sandstone, where reaction with carbonates is not a problem, a pH of 3 requires a strong concentration of urea to start the precipitation reaction and thus a pH of 4 is preferable.
Unfortunately this pH adjustment is impossible, or at best impractical, with the system described in the above patent. The addition of an alkaline agent to a typical aluminum chloride solution in fact produces immediate and inevitable precipitation, and while this precipitation does dissolve slowly over a period of time and lead to a higher pH, its presence requires the use of intermittent mixing techniques. These can only be applied to large volumes of fluid, with operations lasting several days or weeks, particularly as the operations are performed in very restricted areas. The preceding process is valid when fresh water is available. Where the mix water employed is not pure, but rather the classic brine solution found on oilfields, whether brine proper or sea water etc., the initial precipitate formed by the addition of the alkaline agent does not dissolve, even when energetically mixed at high temperatures over a long period. In such cases, which are all too frequent, there is no solution to the problem.
An important step was taken with Main Application FR-A-85.10272, lodged 2 July 1985, which describes the use of an aluminum hydroxychloride. This polymer salt is produced commercially by the electrolysis of aluminum chloride solutions. Its formula in the crystalline form is :

[Al₂(OH)₅Cl, 2.5H₂O]_n

More usually it has the form of a partially neutralized acid aluminum salt, with the general formula :

Al_n(OH)_mX_p

in which X in a mineral or organic anion, or a mixture of mineral and organic anions, with (pxq) + m = 3n, q begin the valency of the anion, the ratio (m/3n) x 100 defining the basicity of the said salt and situated between 30 and 80 %.
Use of an aluminum hydroxychloride as a plugging agent makes it possible to solve the problems encountered with previous techniques and also offers the specialist new possiblities, especially in connection with the effective control of the gelling or precipitation time and monitoring the morphology of the precipitate.
The specialist can refer to the application for above-mentioned French Patent No. 85-10272, the details of which are included here as a reference.
According to this application, it is shown that weak cases other than urea can be used as "acitvators", especially hexametheylenetetramine (activator) in combination with an aluminum hydroxychloride used as a plugging agent.
This invention has now shown that the reaction of the activator is markedly improved by the simultaneous use of sodium cyanate as an activator aid. The two main advantages given by the addition of sodium cycanate are :

1. Better performance of the system at low temperature (< 50.C or < 122. F) and,
2. Greater flexibility in the control of the gelling time.

The specialist will appreciate these two advantages in taking the following two non-limiting examples.
Note: "LOCRON" is an aluminum hydroxychloride corresponding to the above-mentioned formula, which is marketed by the firm of HOECHST AG, Frankfurt, West Germany.

Example A

Different solutions are prepared in a brine containing the "LOCRON"® aluminum hydroxychloride as plugging agent, with the activator being urea in combination with sodium cyanate. the concentration of aluminum hydroxide is 3% by weight.
The concentration of urea and sodium cyanate varies in the manner shown on the attached figure, which gived the gelling time variation according to the cyanate concentration for urea concentrations of between 9 and 3% by weight. The brine is typical of those found in the field.
The gelling time is measured at a temperature of 50.C (122.F). Examination of the curved obtained shows that the gelling time can easily be controlled from about 1 to 2 hours to more than 80 hours. The aluminum hydroxychloride content must be at least 2% by weight. The effective concentration of cyanate is about 500 to 1500 ppm.
The fact of having used a brine with 3% NaCl for the tests, confirms the compatibility of the system with saline mediums which is also an important advantage. In fact the system will operate up to at least 10% NaCl. Beyond this the gel is likely to be less strong.

Example B

The operations are the same as for Example A, with the urea being replaced by hexamethylenetetramine. The results are given in the table below and show that hexamethylenetetramine can largely be used in the same way as urea.

Example C

The following formuala is used, prepared according to the invention :
approx. 3% by weight of aluminum hydroxychloride
approx. 3 - 9% by weight of urea
approx. 1000 ppm of sodium cyanate.

TABLE

Test temperature : 50.C (122.F)
Brine at 1% NaCl

Sodium Cyanate (ppm)	Hexamethylenetetramine (ppm)	Gel time
1000	5000	20 H
1000	4000	40 H
1000	3000	90 H
1100	4000	30 H
1100	3000	60 H
1100	2000	110 H
1200	3500	40 H
1200	3000	60 H
1500	2500	60 H

Claims

Process for delaying and controlling the formation of gels and precipitates derived from aluminum, in which a starting product consisting of a partially neutralized acid salt of aluminum, with the general formula:

Al_n(OH)_m.X_p

(in which X is a mineral or oragnic anion, or a mixture of mineral or organic anions, with (p x q) + m = 3n, q being the valency of the anion, the ratio (m/3n) x 100 defining the basicity of the said salt and situated between 30 and 80%), is submitted to the reaction of a weak-base activator, and characterized by employment in the activator, of sodium cyanate acting as an activator aid.
Process as per Claim 1, characterized by the fact that the said starting product is an aluminum hydroxychloride with the formula 1

Al₂(OH)₅ Cl, 2,5 H₂O
Process as per Claims 1 or 2, characterized by the use of hexamethylenetetramine as activator.
Process as per Claims 1 or 2, characterized by the use of urea as activator.
Process as per any of Claims 1 to 4, characterized by a cyanate concentration of between 500 and 1500 ppm.
Process as per Claims 5, characterized by a urea concentration of 3 to 9%.
Process as per Claim 5 or 6, characterized by an aluminum hydroxychloride concentration in the order of 3%.
Gelling or precipitation retarding compositions obtained by the process of any of Claims 1 to 7.
Applications of the compositions according to claim 8 for the plugging of underground formations, particularly as regards the boring of oil wells or similar.